1. Field of the Invention
The invention relates to a control apparatus for an internal combustion engine and a control method for an internal combustion engine.
2. Description of the Related Art
In an internal combustion engine that operates on blended fuel that contains a plurality of components, the fuel injection amount needs to be corrected based on the component ratio of the fuel. For example, in an internal combustion engine that operates on blended fuel that contains alcohol and gasoline, because the stoichiometric air-fuel ratio for alcohol and that for gasoline are values different from each other, a fuel injection amount corresponding to the concentration of alcohol in the fuel needs to be calculated in order to make the combustion air-fuel ratio equal to the stoichiometric air-fuel ratio. To this end, conventionally, a fuel property sensor that detects a component ratio, such as an alcohol concentration, is provided midway in a fuel supply passage through which fuel is supplied from a fuel tank to an internal combustion engine, and an appropriate fuel injection amount is calculated based on the component ratio detected by the fuel property sensor.
When a fuel tank is refueled with fuel having a different component ratio and thus the component ratio of the fuel in the fuel tank is changed, it is desirable that the fuel injection amount be changed at the time when the component ratio of the fuel that is injected from a fuel injector (hereinafter, referred to as “injection fuel”) changes. In a returnless fuel system having no fuel return pipe for bringing fuel back to the fuel tank from the internal combustion engine, the rate at which fuel travels in the fuel supply passage changes depending on the amount of fuel consumed by the internal combustion engine. For this reason, the time period from when a change in the component ratio is detected by the fuel property sensor to when the component ratio of injection fuel actually changes varies depending upon the amount of fuel consumed by the internal combustion engine.
According to the technology described in Japanese Patent Application Publication No. 11-315744, a fuel passage from a fuel property sensor to a fuel injector is divided into a predetermined number of virtual cells, and information regarding the component ratio at each cell is stored. Each time fuel in an amount corresponding to the size of one cell is consumed by the internal combustion engine, the component ratio information at each cell is moved to the adjacent cell on the downstream side and the component ratio information detected by the fuel property sensor is stored in the most upstream cell. In this way, the component ratio of the injection fuel is estimated, and the fuel injection amount is calculated based on the estimated component ratio.
In an internal combustion engine provided with multiple fuel injectors at respective cylinders, the distance to each fuel injector measured along a fuel passage slightly varies from cylinder to cylinder. Therefore, when the component ratio of the fuel in the fuel tank changes, the component ratio of the injection fuel does not change simultaneously at all the cylinders, more specifically, the change in the component ratio of the injection fuel appears sequentially starting from the cylinder provided with the most upstream fuel injector in the fuel passage. Thus, before the component ratio of the injection fuel is completely changed to a new component ratio at all the cylinders, the component ratio of the injection fuel varies from cylinder to cylinder. In this state, if the internal combustion engine is stopped, the fuel near the fuel injector at each cylinder disperses and is mixed with the fuel near the fuel injector at the adjacent cylinder, and therefore the component ratio of the fuel is increasingly uniformized while the internal combustion engine is kept stopped. That is, the component ratio of the fuel near each fuel injector changes. Therefore, there is a possibility that the component ratio of the fuel that is actually injected from the fuel injector at each cylinder when the internal combustion engine is restarted will be different from the component ratio estimated before the internal combustion engine is stopped. As a result, the fuel injection amount may be excessive or insufficient, which adversely affects the engine startability, etc.